This invention relates to ignition systems for internal combustion engines, and more particularly to apparatus for safely shutting off an internal combustion engine.
Conventional ignition apparatus for internal combustion engines comprises a primary and a secondary winding that are inductively coupled with one another, a spark plug connected across the terminals of the secondary winding, and control switch means for closing a circuit to enable current to flow in the primary winding and for opening that circuit at a time in the engine cycle when the spark plug is to be fired. In a battery ignition system, the closing of the circuit which includes the control switch means allows battery current to flow in the primary winding. In a magneto ignition system, an electromagnetic field is induced in the primary winding by an orbitally moving magnet in cooperation with a fixed ferromagnetic core around which the primary and secondary windings are wound. Closing of the control switch means short-circuits the primary to allow current to flow in it. In either case, opening the primary circuit brings about an abrupt change in a flux field with the secondary winding, thereby inducing a high voltage across the secondary. The conventional control switch means typically has included a pair of hard metal breaker points that were actuated by a mechanism having a cam which rotated in timed relation to the engine cycle. More recently, the control switch means includes a semiconductor device such as a transistor, and a simple means for turning on and turning off the semiconductor device in timed relation to the engine cycle.
Several techniques are known for stopping an internal combustion engine using a safety switch. One such prior art technique is disclosed in U.S. Pat. No. 4,270,509 issued June 2, 1981 to Tharman, and assigned to Briggs & Stratton Corporation, the assignee of the present invention. As depicted in FIG. 6 of U.S. Pat. No. 4,270,509, a safety switch has one terminal connected to the control switch means, with the other safety switch terminal being grounded. When the safety switch is closed the primary winding is grounded, thereby shutting off the engine. An advantage of this so-called ground-to-stop technique is that if the wire connecting the primary winding to the stop switch shorts out to, for example, a metal part on the vehicle, the engine still shuts off. A disadvantage of this technique is that if the wire connecting the primary winding and the stop switch opens or breaks, the stop switch is effectively taken out of the ignition circuit, and cannot be used to stop the engine.
Another prior art technique that uses a single wire connected from the ignition circuit to the stop switch has a stop switch which is opened to shutoff the engine. This so-called open-to-stop technique has one terminal of the stop switch connected in circuit to the control switch means, with the other terminal being grounded. An advantage of this second prior art technique is that if the wire connecting the circuit with the stop switch is opened or breaks, the engine is shut off and a safe failure occurs. However, a disadvantage of this second technique is that if the single wire connecting the circuit to the stop switch is shorted to ground for any reason, the operator will not be able to stop the engine with the stop switch. Another disadvantage of this second technique is that the resistance in the primary circuit is increased, causing possible loss in voltage in the primary circuit.
In summary, both of these prior art techniques have a significant chance of failing in an unsafe mode, preventing the stop switch from shutting off the engine.